Keep in mind that the sugar is already in solution. Unless you're already at the limit of sugar solubility in the water, hotter water won't matter. And in a mash, you're nowhere near that limit. There is no solid sugar to be dissolved during the sparge, since the sugar is all in solution when it is created. The solubility of maltose in water at mash temps is about 66.7 % by weight (2 lb of maltose will dissolve in 1 lb of water, (ref:http://chestofbooks.com/food/science...er-gillis.html), and this is equivalent to an SG in excess of 1.300.
But like you, I sparge with hot water...like 185-195 hot. Because the pH is fine, there is no tannin extraction. And the extra heat gets me the last little bit of conversiuon I might otherwise miss.
I totally agree with this. To put it another way, the viscosity of wort is nowhere near that of honey. The consistency of wort in the mash has more similarity to water than it does to honey, or to molasses, or liquid malt extract, etc. It's already very well dissolved in a lot of water. So, if you heat it up, it's not really helping make it more fluid or "more dissolved" (really no such thing); it's already plenty fluid and 100% dissolved.
The only real reason, then, for anyone to heat up the sparge water is to save some time later in how much time it takes to bring all the wort to a boil. By adding 190 F water, or even boiling water at 212 F(!), you're basically preheating the wort, with no detrimental effects. Hell...... I think I'll start using boiling water from now on. And why not! There's no drawbacks at all, as long as mash pH is reasonable in the low 5's, which mine is.
Yep, we talked me into it. From now on, I shall use boiling water for all my sparges. I'm dead serious.